1. Field of the Invention
The present invention relates to a plating apparatus for applying copper plating or the like to the surface of a wafer in a semiconductor fabrication process.
2. Description of the Related Art
Copper traces in a semiconductor device are fabricated by e.g. damascene, in which copper is embedded into trenches formed in the surface of a semiconductor substrate, to form so-called trenched traces. Here, for the sake of forming a copper layer all over the surface thereof so as to fill the trenches, plating is applied instead of sputtering or the like.
FIG. 1 is a schematic diagram showing a conventional wafer plating apparatus. A plating bath 1 contains a tubular, closed-bottomed partitioning member 2 which is smaller than the plating bath 1. The partitioning member 2 is arranged to be spaced from the plating bath 1. A pipe 10a connected to a plating solution supply port of a plating solution circulator 9 is introduced into a space enclosed by the partitioning member 2. The extremity of a pipe 10b connected to a plating solution circulation inlet of the plating solution circulator 9 is introduced to a space between the plating bath 1 and the partitioning member 2. By this means, a plating solution 11 is supplied to the inside of the partitioning member 2 before it flows over the top of the partitioning member 2 to the bottom of the plating bath 1.
In the partitioning member 2, an anode electrode 4 is arranged with its surface horizontal via an insulative supporting member 3. Above this anode electrode 4 is arranged a wafer holder 5. This wafer holder 5 fixes a wafer 6 with the surface horizontal so as to face the anode electrode 4. This wafer 6 is in contact with hook-like cathode terminals 7 at several portions. A power supply 8 supplies a negative potential and a positive potential to the cathode terminals 7 and the anode electrode 4, respectively.
In the wafer plating apparatus configured as described above, the plating solution, such as a copper sulfate aqueous solution, is supplied from the plating solution circulator 9 to the inside of the partitioning member 2 through the pipe 10a. The plating solution flowing over the top of the partitioning member 2 is returned from the bottom of the plating bath 1 to the plating solution circulator 9 through the pipe 10b. Meanwhile, the wafer 6 has a thin copper seed layer formed on its surface by sputtering or the like, before loaded in the plating bath 1. Then, the power supply 8 applies a prescribed voltage across the anode electrode 4 and the wafer 6 (the cathode terminals 7) to form an electric field directed from the anode electrode 4 (+) to the wafer 6 (xe2x88x92). This electric field in the plating solution deposits, for example, copper ion in the plating solution onto the surface of the wafer 6, thereby forming a plating layer on the surface of the wafer 6. The amount of this copper deposition depends on the current densities on the wafer surface.
In this conventional wafer plating apparatus, electric lines of force directed from the anode electrode 4 to the seed layer of the wafer 6 are formed in the region where the anode electrode 4 and the wafer 6 are opposed to each other. These electric lines of force are, however, nonuniform within the surface of the wafer 6, thereby causing unevenness in copper plating thickness. The reason for this is that the cathode terminals 7 and the seed layer of the wafer 6 contact with each other on the periphery of the wafer. The power supply voltage is initially supplied to the seed layer on the periphery of the wafer, and therefore the voltage within the wafer surface becomes nonuniform due to voltage drops in the seed layer. Besides, the configuration of the plating bath and the electrical properties of the plating solution cause a change in the electric field distribution between the anode electrode 4 and the wafer 6. This results in nonuniform current densities within the wafer surface, thereby generating unevenness in plating thickness. Thus, a plating layer thinner at the wafer center and thicker on the wafer periphery is formed on the wafer surface.
This unevenness in plating thickness deteriorates the uniformity of the wiring resistance within the surface when the traces are formed by damascene. Moreover, in the conventional plating apparatus, correction is difficult to make when subtle fluctuations of the plating bath and/or changes of the plating solution deteriorate the uniformity of the plating thickness within the wafer surface.
For the sake of maintaining the plating layer at a constant film thickness, an electroplating method has been proposed in which a shielding electrode is arranged in the vicinity of a cathode electrode, and the electric current flowing from an anode electrode to the cathode electrode and the electric current flowing from the anode electrode to the shielding electrode are supplied by separate constant-current sources (Japanese Patent Laid-Open Publication No.Hei 9-157897).
This method, however, has a problem of lower throughput since some electric current is wasted due to the presence of the current flowing to the shielding electrode, aside from the current for plating an object to be plated.
Moreover, for the purpose of uniformizing the current density distribution within the plating area, a plating-film forming method has been proposed in which a film thickness adjusting plate having an opening is arranged on the paths of the plating currents to narrow the current paths so that the currents to the periphery of an object to be plated take longer paths to prevent the electric field concentration on the periphery of the object to be plated (Japanese Patent Laid-Open Publication No.Hei 8-100292).
However, this method also has the problem of lower throughput because it improves the plating uniformity by reducing the current densities on the area apt to greater plating thicknesses.
It is thus an object of the present invention to provide a plating apparatus which can create a uniform plating layer on the surface of an object to be plated with high productivity, and can control the electric field in the vicinity of the object to be plated independent of fluctuations of the plating bath and/or changes of the plating solution, so as to control the plating thickness distribution.
A plating apparatus according to the present invention comprises: a plating bath for containing a plating solution; a holder for holding an object to be plated in the plating solution; an electrode opposed to the object to be plated within the plating solution; a first power supply for supplying a negative potential to the object to be plated and a positive potential to the electrode; an electric field adjusting member of conductive material, interposed between the object to be plated and the electrode within the plating solution; and a second power supply for supplying a potential to the electric field adjusting member. Here, the electric field adjusting member adjusts the distribution of the electric lines of force directed from the electrode to the object to be plated, by using the potential supplied from the second power supply.
In this plating apparatus, the electric field adjusting member may be a disk-like auxiliary electrode having a diameter smaller than that of the object to be plated, being interposed between the object to be plated and the electrode. Here, the second power supply supplies a positive potential equal to or lower than that of the electrode to the auxiliary electrode. Besides, the auxiliary electrode preferably has a plating solution hole formed therethrough along the direction of thickness. Moreover, the surface opposed to the object to be plated, of the auxiliary electrode may be configured to rise at the center and sink to the periphery of the auxiliary electrode. In this case, the auxiliary electrode may be thick at the center thereof and thin on the periphery thereof.
Another plating apparatus according to the present invention comprises: a plating bath for containing a plating solution; a holder for holding an object to be plated in the plating solution; an electrode opposed to the object to be plated within the plating solution; and a power supply for supplying a negative potential to the object to be plated and a positive potential to the electrode. Here, the surface opposed to the object to be plated, of the electrode rises at the center thereof toward the object to be plated.
In the present invention, the auxiliary electrode or the like shortens the distance between the center of the object to be plated, apt to smaller plating thicknesses, and the electrode to raise the current density in this area. This increases the electric lines of force in the area, thereby enhancing the uniformity in plating thickness. As a result, a uniform plating layer can be formed with higher productivity.
The nature, principle, and utility of the invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.